In vehicle AC devices employed in electric automobiles, hybrid vehicles, and the like, for example, a PTC heater that uses a positive temperature coefficient thermistor element (called a “PTC element” hereinafter) as a heat generating element is known as an example of a heat source for heating (see Patent Documents 1 and 2, for example).
Electrification control of a PTC element is carried out by turning a switching element such as an insulated gate bipolar transistor (IGBT) on and off, and in the case where the switching element has malfunctioned, a current will continue to flow to the PTC heater. It is therefore necessary to detect an on-state malfunction in the switching element.